The present invention relates to an apparatus and method for driving a matrix liquid crystal display (LCD) panel, and more particularly, to an apparatus and method for driving a matrix LCD panel and the method thereof, whereby the brightness ratio of liquid crystal is improved and crosstalk is prevented to give improved response characteristics.
In general, an LCD can be driven by a single-line sequential driving method (called alto-pleshko technology) by which each scanning electrode is sequentially driven, or a plural-line simultaneous driving method by which a plurality scanning electrodes are simultaneously selected and the driving signal for the corresponding data electrodes is processed and applied by a data processor. In addition, an orthogonal function driving method and an active driving method are adopted for the driving of a finer screen such as that used in a television or the like.
FIGS. 3A and 3B are waveform diagrams showing a scanning electrode driving signal and a data electrode driving signal for a conventional matrix LCD panel, respectively, and FIG. 5 is a graph showing light transmittance (T) of general liquid crystal according to the applied voltage (V) thereto.
According to the above single-line sequential driving method, when a scanning electrode Y1 is selected, in order to drive liquid crystal cells of the line corresponding thereto, data electrode driving signals X1 through XM are controlled to be simultaneously applied. As shown in FIG. 3A, this operation is repeated sequentially during one frame period until Y1 through YN are selected. In FIG. 3B, the dotted line represents a specific data electrode driving signal X.sub.M, and the solid line represents state changes of scanning electrode driving signal Y.sub.N, from the non-selection state to the selection state and back to the non-selection state. According to the single line sequential driving method, the number of lines is determined by a duty principle. For example, when sequentially driving 240 scanning electrodes, the scanning electrode driving signal has a duty cycle of 1/240.
FIGS. 2A to 2C are waveform diagrams for illustrating the operation of a conventional matrix LCD, which is driven by the single-line sequential driving method. FIG. 2A shows a scanning electrode driving signal and FIG. 2B shows a data electrode driving signal. Here, the scanning electrode driving signal and the data electrode driving signal are inverted in polarity in a period of one frame to then be applied, which is to prolong the life of an LCD by preventing liquid crystal deterioration. FIG. 2C shows a voltage waveform directly applied to the liquid crystal by the scanning electrode driving signal and the data electrode driving signal, the amount of which is obtained by the difference (A B) between voltages of the scanning electrode driving signal and the data electrode driving signal.
However, in a single-line sequential driving method, the more lines a display device has, the less time per line is required, thereby increasing the comparative magnitude of a driving signal in accordance with the duty concept. That is to say, as the number of lines increases, the time for a given line to be selected is reduced. Thus, the liquid crystal cells located at each line are not fully driven, which results in a deterioration of the response speed and the response characteristics of the liquid crystal itself.
Another problem is crosstalk which is an inherent display characteristic of LCDs having a simple matrix structure. FIG. 4A shows an example of images formed on a conventional matrix LCD panel; and FIGS. 4B through 4E are waveform diagrams showing the change in the effective voltage level of a liquid crystal pixel, caused by a waveform differential according to the data electrode driving signal at a non-selection scanning electrode of the matrix LCD panel having the above example of images formed thereon.
As shown in FIGS. 4B through 4E, if the data electrode driving signal undergoes a transition from a high state to a low state, or vice versa, the differential waveform is produced at a non-selected scanning electrode, thereby resulting in an increase or decrease in the effective voltage level which is directly applied to the liquid crystal.
As described above, if the effective voltage level directly applied to the liquid crystal is increased or decreased and then the state of a data electrode driving signal is not changed, in contrast to the case where a waveform differential is not induced for the non-selected scanning electrode driving signal, an error in the effective voltage levels applied to the liquid crystal is produced, which is a cause of crosstalk generated in an LCD.
Meanwhile, another method of driving liquid crystal is a plural-line simultaneous driving method by which a predetermined number of scanning electrodes form subgroups, a scanning electrode driving signal is applied in subgroup units, and the data electrode driving signal corresponding thereto is applied. A detailed exemplary operation of the plural-line simultaneous driving method is disclosed in a paper entitled "Optimal Row Functions and Algorithms for Active Addressing" (see Digest SID '93, pp89-92) and is mainly adopted to an LCD used for its high-speed response characteristics.
However, according to the above plural-line simultaneous driving method, the pulse width of a scanning electrode driving signal is reduced depending on the increase in the number of scanning lines, and the amplitude of the scanning electrode signal is increased. Also, since the data electrode driving signal has plural values and due to the large amplitude of the data signal, the voltage waveform induced for the scanning electrode driving signal experiences overshoot when the state of the data electrode driving signal changes, whereby the error in the effective voltage level directly applied to the liquid crystal becomes greater.
Also, in large LCD panels, since the resistance of the transparent electrode made of a material such as indium tin oxide (which is the material constituting the scanning electrodes in an LCD panel) is high, a delay in the scanning electrode driving signal is generated between the two ends of the electrodes in the liquid crystal panel. The delay of the scanning electrode driving signal causes errors in the effective voltage levels directly applied to the liquid crystal, which results in screen heterogeneity.
A solution to the above problems is disclosed in Korean Patent Application No. 93-29043 (by the applicant of the instant invention). The contents thereof will now be discussed with reference to FIG. 1 which shows a conventional matrix LCD panel and a driving apparatus therefor.
As shown in FIG. 1, the driving apparatus of a conventional matrix liquid crystal display device includes an LCD panel 5 wherein a matrix of scanning electrodes and data electrodes is formed and liquid crystal is arranged in the intersections thereof and data electrode, a controller 1 for receiving a video data signal and vertical and horizontal synchronization signals and generating a driving timing control signal and a data signal, a driving voltage generator 2 for generating a voltage signal of each level required for a driving signal to send to a data electrode driver 3 and a scanning electrode driver 4, a scanning electrode driver 4 to which the driving timing control signal is input from the controller 1 and to which the voltage signal of each level is input from the driving voltage generator 2, for generating a scanning electrode driving signal, and a data electrode driver 3 to which a driving timing control signal and data signal are input from the controller 1 and to which the voltage signal of each level is input from the driving voltage generator 2, for generating a data electrode driving signal.
The driving apparatus and method for the above matrix LCD panel will now be described.
Proper circuit operation requires an input video data signal and vertical and horizontal synchronization signals in the form of a composite video signal input to the controller 1 which thereby determines a driving timing control signal and a high or low data signal to be applied to the data electrodes of the LCD panel 5, according to either the plural-line simultaneous driving method or a single-line sequential driving method. The driving timing control signal is generated such that the driving signals applied to adjacent scanning electrodes of the LCD panel 5 overlap each other, as shown FIG. 6, wherein a waveform diagram illustrates the scanning electrode driving signals for driving a matrix LCD in one embodiment of the prior application. The data signal is generated so as to maintain an intermediate level during the time corresponding to the overlapping interval of the scanning electrode driving signal when the signal is changed from a high state to a low state, or vice versa, as shown in FIG. 7 wherein a waveform diagram illustrates a data electrode driving signal overlapping a scanning electrode driving signal for driving a matrix LCD in the above embodiment of the prior application.
FIG. 9 shows scanning electrode and data electrode driving signals according to the plural-line simultaneous driving method. In the example described herein, the scanning electrode driving signals of three lines are formed into a subgroup and orthogonal function values are applied to each line.
In the driving voltage generator 2, a voltage signal of each level necessary for the driving electrode driving signals applied to the scanning electrodes and data electrodes of the LCD panel 5 and voltage signals of an intermediate level necessary for data electrode driving signals are generated, to then be output to the data electrode driver 3 and scanning electrode driver 4.
In the scanning electrode driver 4, the corresponding voltage level of the driving voltage generator 2 is selected from the driving timing control signal input from the controller 1 and then a scanning electrode driving signal is generated. The scanning electrode driving signals are mutually overlapped for a predetermined period to then be sequentially applied to the scanning electrode of the LCD panel 5 by the driving timing control signal, as shown in FIGS. 6 and 9.
The "high" period of a scanning electrode driving signal is increased by as much as the overlap between the scanning electrode driving signals, while the "high" level thereof is decreased. In other words, since the selection time applied to the scanning electrode of the liquid crystal panel is increased, the selection ratio of the scanning data electrode is also increased (an increased scanning ratio), thereby improving the speed of the liquid crystal response.
In the data electrode driver 3, video data signals input from the controller 1 are stored in parallel. Thereafter, a voltage level corresponding to each video data signal is selected as one of the voltage signals input from the driving voltage generator 2. The voltage signals for driving the respective selected data electrodes are simultaneously applied to the data electrodes of LCD panel 5 when scanning electrode driving signals are applied to the scanning electrodes of LCD panel 5.
The data electrode driving signal is selected from among the voltage signals input from the driving voltage generator 2 by data electrode driving time control signal from the controller 1, so as to maintain an intermediate level for the duration of the overlap interval between a selection pulse of a scanning electrode driving signal and another selection pulse of an adjacent scanning electrode driving signal, as shown in FIG. 7.
As described above, when the data electrode driving signal goes from a low state to a high state, or vice versa, the signal level temporarily maintains an intermediate level which is lower than the pixel-switching level. This is for reducing a waveform differential induced for a non-selected scanning electrode by reducing the magnitude of the signal changes of the data electrode driving signal.
If the waveform differential induced for the scanning electrode driving signal is decreased, the error in the effective voltages directly applied to the liquid crystal is also reduced, thereby reducing the generation of crosstalk.
The scanning electrode driving signal is applied in one line or subgroup unit by the scanning electrode driver 4, and a data electrode driving signal is controlled so as to be applied to the data electrode of LCD panel 5 whenever the scanning electrode driving signal is applied to the LCD panel 5. Thus, each liquid crystal cell of LCD panel 5 is driven to a proper level, which results in displaying the desired picture information.
FIGS. 8A and 8B are waveform diagrams of the data electrode driving signal overlapping with the scanning electrode driving signal, in another embodiment of the prior application.
Accordingly, the above problems may be solved by merely overlapping scanning electrode driving signal. However, a problem of reduced contrast ratio still remains.